1. Field of the Invention
The present invention generally relates to the art of microelectronic circuit fabrication, and more specifically to a microelectronic circuit leadframe package such as a Tape-Automated-Bonding (TAB) package including integral support members for outer lead form protection during mounting on a printed circuit board.
2. Description of the Related Art
Tape-Automated-Bonding (TAB), also known as Chip-on-Tape (COT), is a system for assembling, shipping and mounting microelectronic circuit chips or dies that are integrally attached and interconnected to a flexible leadframe. TAB enables a large number of chip/leadframe modules to be automatically fabricated with high speed and precision in an economical manner.
The TAB system fabricates a large number of identical leadframe metallization patterns on a roll of polyimide tape that is movable between reels through the required processing stations. The individual patterns are cut from the roll to form leadframe packages. Microelectronic integrated circuit dies are mounted on the packages and interconnected with the leadframes, and the resulting TAB/COT modules are inserted into individual plastic carriers for shipping to end users.
A typical COT 10 is illustrated in FIG. 1, and comprises a flexible polyimide sheet or film 12 that is formed with peripheral perforations 14 to enable reel-to-reel transport of the roll from which the film 12 was cut using a sprocket (not shown). The film 12 is formed with a central hole 16 for receiving a microelectronic circuit die or chip 18.
The COT 10 further comprises a leadframe 20 that is formed on the film 12 as a metallization pattern. The leadframe 20 includes a plurality of leads 22 that extend laterally outward from bonding pads or terminals 24 (see FIG. 3) on the chip 18 to test pads 26 that are located adjacent to the periphery of the film 12. Each lead 22 includes an inner portion 22a that extends from the chip 18 to the edge of the hole 16, an intermediate portion 22b that extends under an optional protective epoxy layer 28, an outer portion 22c and a peripheral portion 22d that interconnects with the respective test pad 26.
The film 12 is further formed with excise apertures or holes 30 through which the outer portions 22c of the leads 22 extend. After the COT 10 has been received by the end user, it is tested using the test pads 26. The COT 10 is then subjected to further processing including excising the inner portion of the leadframe 20 and the chip 18 from the film 12. This is performed by severing the outer portions 22c of the leads 22 at the outer edges of the holes 30, and cutting webs 32 that connect the portion of the film 12 laterally inward of the holes 30 from the remainder of the film 12.
A module 34 that results from the excise step is illustrated in FIG. 2, and consists of the leadframe 20 with the peripheral portions 22d of the leads 22 removed, and the chip 18. The outer portions 22c of the leads 22 are redesignated as 36.
Although only a few leads are shown in FIGS. 1 and 2 for clarity of illustration, the leadframe 20 of an actual COT 10 includes a much larger number of leads, typically hundreds, that are spaced from each other by a pitch of less than 100 micrometers. The outer leads 36 as viewed in FIG. 2 are extremely thin and fragile, and can be easily deformed and/or damaged.
For this reason, the COT 10 cannot be handled or shipped in the configuration illustrated in FIG. 2 with the leads 36 free. It is necessary to maintain the COT 10 in the form illustrated in FIG. 1 until it is to be mounted on a printed circuit board (PCB) or other surface, including providing a protective carrier in which the COT 10 is retained for shipping. The carrier is necessarily larger and weighs more than the COT 10. This introduces increased shipping charges, in addition to the cost of the carrier itself.
Some leadframe packages have leads that are sufficiently large and strong that they can be bent into a conventional "gull-wing" or other suitable shape and provide sufficient rigidity for unsupported mounting on a PCB. The thin leads of the COT 10, however, are not able to support the weight of the assembly 34, and will collapse if an attempt is made to surface mount the module 34 of FIG. 2 in this manner.
For this reason, the COT 10 is conventionally mounted on a PCB 38 or other supporting surface formed with conjugate bonding pads or terminals 40 using a "hot bar" process as illustrated in FIG. 3. The COT 10 is moved to an excise station, while remaining in its protective carrier, where the leadframe 20 and chip 18 are removed from the outer portion of the film 12 and picked up by a vacuum arm. During this excise operation, the leads 36 are bent into a gull-wing shape as illustrated in the drawing.
The module 34, as excised from the COT 10 and having the leads formed into the gull-wing shape, is then placed by the arm 42 onto the PCB 38 with the leads 36 in contact with the terminals 40. A hot bar 44 presses the leads 36 against the terminals 40 and causes solder that was previously coated on the terminals 40 to reflow and ohmically adhere the leads 36 to the terminals 40.
Although capable of providing the required function of TAB mounting, the hot bar apparatus is expensive and slow, and is therefore impractical for small scale users to acquire. In addition, the conventional system for packaging, shipping and mounting COTs suffers from other drawbacks and disadvantages.
Due to the fragility of the outer leads, reworking of mounted COTs to correct, for example, defective solder joints is impractical. Removal of defective COTs after mounting is also difficult, since the fragile leads are easily broken and can remain attached to the PCB. It is also excessively difficult to test a COT after it has been excised from the package as illustrated in FIG. 1 and assumes the form illustrated in FIG. 2 due to the small size, extreme flexibility and fragility of the leads 36.